# Psychrometric Charts

## What is Psychrometric Chart?

A Psychrometric Chart is an important tool for HVAC engineers to carry out heat load or cooling load calculations and find solutions to various air condition related problems.

Psychrometric charts are graphic representations of the psychrometric properties of air. By using psychrometric charts HVAC engineers can graphically analyze different types of psychrometric processes and find solution to many practical problems without having to carry out long and tedious mathematical calculations.

The psychrometric chart looks complicated with vast numbers of lines and curves in it, but is very easy to understand if you know the basic properties of air.

You will also understand its worth when you actually use it considering the fact that you won’t have to use any formulas to find the properties of air in different conditions, all you will have to know is two parameters of air and the rest are easily found on the chart.

What does a Psychrometric chart show?

A psychrometric chart shows several common properties of air at different conditions. The psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters.

The air properties on a psychometric chart encompass:

Dry-bulb temperature (DBT): DBT of an air sample, as determined by an ordinary thermometer. It is typically plotted as the abscissa (horizontal axis) of the graph

Wet-bulb temperature: WBT is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel.

Humidity ratio: It is the proportion of mass of water vapor per unit mass of dry air at the given conditions (DBT, WBT, DPT, RH, etc.). It is also known as moisture content or mixing ratio. It is typically plotted as the ordinate (vertical axis) of the graph. For a given DBT there will be a particular humidity ratio for which the air sample is at 100% relative humidity: the relationship reflects the physics of water and air and must be determined by measurement.

Specific volume: It is the volume of the mixture (dry air plus the water vapor) containing one unit of mass of “dry air”.

Dew point temperature: DPT is the temperature at which a moist air sample at the same pressure would reach water vapor “saturation.” At this point further removal of heat would result in water vapor condensing into liquid water fog. The dew point temperature is measured easily and provides useful information.

Enthalpy: It is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within.

Relative Humidity (RH): RH is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. Mole fraction is a way of expressing the composition of a mixture. The mole fraction of each component is defined as its amount of substance divided by the total amount of substance in the system. RH is dimensionless, and is usually expressed as a percentage. Lines of constant RH reflect the physics of air and water: they are determined via experimental measurement.